I completely disagree with Dan Gilbert's letter.
You can't vote a trade down because it "helps a team too much". As Herm Edwards would probably say: "HELLO?! You make trades to improve your team and situation! Not to just trade!"
Where is this threshold line that distinguishes a trade that makes a team better enough to be acceptable and a trade that makes a team "too" much better?
The only appeal you can make to veto a trade is if the trade is not in the best interest of another team. That means one team is getting way better and another team is getting hosed. That throws off the competitive balance of the league and hints at possible collusion. That's the only reasonable gripe you can have with a trade.
Whether or not that is your opinion is up to you (I believe NO did not improve their overall situation best here). But that is the only case you can make for a trade being vetoed. Not because one team gets "too good". That should be the objective of teams when making trades. Not a dissuading point.